54. : On the UNEQUAL 
the marine acid, does not difperfe the red and violet rays quite 
fo much. The green ray, as in the marine acid, is found 
among the'more refrangible rays ; but it approaches nearer to 
the place of the mean refrangible ray in the nitrous acid than 
in the marine. The green ray is alfo nearer to the place of the 
mean refrangible ray, than it is in effential oils or in faturated 
metallic folutions ; and therefore the nitrous acid appears by 
thefe experiments to difperfe the feveral orders of rays more 
nearly in the fame proportion in which crown-glafs does, than 
any uncompounded difperfive medium, and would, I have no 
doubt, do fo exaétly, if flightly impregnated with mercury, 
though this I have not tried. 
THE vitriolic acid is fcarcely to be claffed among difperfive 
mediums. The following experiment is the laft I made on the 
fubject. Ina very good object-glafs, of that kind before de- 
{cribed, in which fpirit of wine is one of the mediums em- 
ployed, I fubftituted fucceflively for this fpirit the vitriolic 
acid and. a folution of fixed alkaline falt, both of them of near- 
ly the fame mean refractive denfity as the fpirit of wine. Thefe 
three fluids, although they differ fo widely in their chemical 
properties, have their optical properties fo nearly alike, that I 
found it dificult to determine which was the medium em- 
ployed. For when the fecondary colour is not corrected, as 
was the cafe in this object-glafs, the change of colour produced 
in the green and purple fringes to render it apparent, muft be 
confiderable, a flight fhade of difference not being eafily di- 
ftinguifhable.' I therefore repeated the trial with an objedt- 
glafs, in which this green and purple light is totally removed 
and then both the vitriolic acid, and the folution of fixed al- 
kali, when of equal mean refractive denfity with fpirit of wine, 
appeared very fenfibly more difperfive than the fpirit. The 
difference in this refpect between the acid and the alkali was 
fcarcely to be diftinguifhed ; and the effect of a folution of 
cauftic alkali appeared to be nearly the fame as that of mild 
alkali 
